A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon substrate). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
European patent application EP-A-1 037 117 discloses a level sensor in a lithographic projection apparatus. Using this sensor a height map of a substrate in a lithographic map is determined by using grating optics and polychromatic radiation in the wavelength range of 600-1050 nm.
The level sensor of EP-A-1 037 117 uses a separate capture spot and detection spot, wherein the detection spot is used to determine the height map, and wherein the capture spot is used to capture the surface of the substrate within a desired measurement range of the detection spot, e.g. a linear or linearized measurement range.
The detection spot and capture spot are formed by illuminating a projection grating, wherein the projection grating comprises a grating for forming the detection spot and an additional aperture for forming a capture spot ahead of the detection spot on the substrate.
The detection spot is reflected by the substrate and is incident on a detection grating which is essentially a copy of the grating of the projection grating for determining the height map of the substrate surface.
The capture spot formed by the projection grating passes the detection grating, where it is incident on three separate detection regions. Two of the detection regions are set to high and one is set to low. The output of the low detection region is subtracted from those of the high regions. The capture spot detection regions are arranged so that when the substrate surface is at a zero position, the capture spot falls equally on the high and low detection regions and the subtracted output is zero. Away from the zero position, more of the capture spot will fall on one of the detection regions than the other and the subtracted output will increase in magnitude with its sign indicating whether the substrate is too high or too low.
However, the current level sensors as described above are not always satisfactory.